We’ve created a world with 300 exabytes of human-made information. Try processing all that.

Today, each of us individually generates more information than ever before in human history. Our world is now awash in an unprecedented volume of data. The trouble is, our brains haven’t evolved to be able to process it all.

As former Boeing scientist and New York Times writer Dennis Overbye notes, this information stream contains “more and more information about our lives—where we shop and what we buy, indeed, where we are right now—the economy, the genomes of countless organisms we can’t even name yet, galaxies full of stars we haven’t counted, traffic jams in Singapore, and the weather on Mars.” That information “tumbles faster and faster through bigger and bigger computers down to everybody’s fingertips, which are holding devices with more processing power than the Apollo mission control.”

Just trying to keep our own media and electronic files organized can be overwhelming. Each of us has the equivalent of over half a million books stored on our computers, not to mention all the information stored in our cell phones or in the magnetic stripe on the back of our credit cards. We’ve created a world with 300 exabytes (300,000,000,000,000,000,000 pieces) of human-made information. If each of those pieces of information were written on a 3-by-5-inch index card and then spread out side by side, just one person’s share—your share of this information—would cover every square inch of Massachusetts and Connecticut combined.

Our brains have the ability to process the information we take in, but at a cost: We can have trouble separating the trivial from the important, and all this information processing makes us tired.

Neurons are living cells with a metabolism; they need oxygen and glucose to survive, and when they’ve been working hard, we experience fatigue. Every status update you read on Facebook, every tweet or text message you get from a friend, is competing for resources in your brain with important things like whether to put your savings in stocks or bonds, where you left your passport, or how best to reconcile with a close friend you just had an argument with.

The processing capacity of the conscious mind has been estimated (by the researcher Mihaly Csikszentmihalyi and, independently, by Bell Labs engineer Robert Lucky) at 120 bits per second. That bandwidth, or window, is the speed limit for the traffic of information we can pay conscious attention to at any one time.

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In 2011, Americans took in five times as much information every day as they did in 1986.

While a great deal occurs below the threshold of our awareness, and this has an impact on how we feel and what our life is going to be like, in order for something to become encoded as part of your experience, you need to have paid conscious attention to it.

What does this bandwidth restriction—this information speed limit—mean in terms of our interactions with others? In order to understand one person speaking to us, we need to process 60 bits of information per second. With a processing limit of 120 bits per second, this means you can barely understand two people talking to you at the same time. Under most circumstances, you won’t be able to understand three people talking at the same time. We’re surrounded on this planet by billions of other humans, but we can understand only two at a time at the most! It’s no wonder that the world is filled with so much misunderstanding.

With such attentional restrictions, it’s clear why many of us feel overwhelmed by managing some of the most basic aspects of life. Part of the reason is that our brains evolved to help us deal with life during the hunter-gatherer phase of human history, a time when we might encounter no more than a thousand people across the entire span of our lifetime. Walking around midtown Manhattan, you’ll pass that number of people in half an hour.

Attention is the most essential mental resource for any organism. It determines which aspects of the environment we deal with, and most of the time, various automatic, subconscious processes make the correct choice about what gets passed through to our conscious awareness. For this to happen, millions of neurons are constantly monitoring the environment to select the most important things for us to focus on.

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The processing capacity of the conscious mind has been estimated at 120 bits per second.

These neurons are collectively the “attentional filter.” They work largely in the background, outside of our conscious awareness. This is why most of the perceptual detritus of our daily lives doesn’t register, or why, when you’ve been driving on the freeway for several hours at a stretch, you don’t remember much of the scenery that has whizzed by: Your attentional system “protects” you from registering it because it isn’t deemed important. This unconscious filter follows certain principles about what it will let through to your conscious awareness.

The attentional filter is one of evolution’s greatest achievements. In nonhumans, it ensures that they don’t get distracted by irrelevant things. Squirrels are interested in nuts and predators and not much else. Dogs, whose olfactory sense is 1 million times more sensitive than ours, use smell to gather information about the world more than they use sound, and their attentional filter has evolved to make it so. If you’ve ever tried to call your dog while he’s smelling something interesting, you know that it is very difficult to grab his attention with sound—smell trumps sound in the dog brain.

No one has yet worked out all of the hierarchies and trumping factors in the human attentional filter, but we’ve learned a great deal about it. When our protohuman ancestors left the cover of the trees to seek new sources of food, they simultaneously opened up a vast range of new possibilities for nourishment and exposed themselves to a wide range of new predators. Being alert and vigilant to threatening sounds and visual cues is what allowed them to survive; this meant allowing an increasing amount of information through the attentional filter.

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Our brains do have the ability to process the information we take in, but at a cost.

Humans are, by most biological measures, the most successful species our planet has seen. We have managed to survive in nearly every climate our planet has offered (so far), and the rate of our population expansion exceeds that of any other known organism. Ten thousand years ago, humans plus their pets and livestock accounted for about 0.1% of the terrestrial vertebrate biomass inhabiting the earth; we now account for 98%. Our success owes in large part to our cognitive capacity, the ability of our brains to flexibly handle information. But our brains evolved in a much simpler world with far less information coming at us. Today, our attentional filters easily become overwhelmed.

Successful people—or those who can afford it—employ layers of other people whose job it is to narrow their own attentional filters. Corporate heads, political leaders, movie stars, and others whose time and attention are especially valuable have a staff around them who are basically extensions of their own brains, replicating and refining the functions of the prefrontal cortex’s attentional filter. For the unlucky rest of us, though, the fact is that paying attention will continue to cost our mental faculties a great deal.